Waterflood method



Feb. 4, 1964 D. L. ARCHER 3,120,262

WATERFLOOD ME'IHOD Filed Nov. 13, 1962 ORIGINAL ORIGINAL OIL-lN-PLACEWELL 5 OIL-IN-PLAOE ELL 5 FIG.I FIG. 2

WATER BANK WATER WELL A CO2- ENRICHED OIL wELLA ORIGINAL BANKOIL-lN-PLACE co ENRICHED CO2 OIL PROOucEO OlL-lN-PLACE INJE TEDOlL-lN-PLACE EL 8 FIG L 2 -ENRICHED OIL WELL A ORIGINAL OIL-lN-PLACE coINJECTED o|L..|N..p c

PRODUCED ORIGINAL OIL-IN-P AGE w LL 8 AREA OF ADDITIONAL L2 OIL RECOVERYBY PROPOSED METHOD AREA SWEPT BY WATER ORIvEN CO2 PROcEss ORIGINALOlL-IN-PLACE WELL A DUANE L. ARCHER, INVENTOR.

BYM- by. $57

A T T'ORNE Y United States Patent can Petroleum Corporation, Tulsa,01th., a corporation of Delaware Filed Nov. 13, 1962, Ser. No. 237,10513 Claims. (Cl. 166-9) The present invention is concerned with animproved process for recovery of oil from an underground reservotr. Moreparticularly, it relates to an improved secondary method of oil recoverybased upon the general phenomenon of reducing the initial viscosity ofthe oil by dissolving therein an oil-soluble gas and following with awater or similar flood whereby the oil of reduced viscosity is recoveredin increased amounts. Specifically, the process of my invention.although involving the idea of reducing the viscosity of the oil inplace prior to recovery. concerns primarily a sequence of steps by whichthe aforesaid phenomenon is utilized to the fullest CK Cl'll.

While the following description is directed chiefly to the use of carbondioxide as the oil-soluble gas employed m my invention. obviously othergases. such as methane. sulfur dioxide. hydrogen sulflde. carbonmonoxide, and the like. or mixtures of one or more of said gases with orwithout carbon dioxide are contemplated.

The elilciency of oil production from underground reservoirs by anysecondary recovery process is a funcllOll of the ultimate recovery perbarrel of oil in place at the beginning of the process. Such efiicicncydepends upon three principal variables: the vertical sweep cfflciency.the horizontal sweep efllciency. and the displacement eflicicncy. Thisinvention. however. is concerned primarily with the horizontal sweepefliciency and. to some extent, with improvement in the displacementcfliciency which relates to the volume of driven fluid actuallyrecovered from a particular unit of a reservoir by displacement with adriving fluid.

l have found in comparing an ordinary water drive or flood with aprocess involving flrst injecting an oilsoluble gas. such as carbondioxide. into the reservoir oil followed by water drive. the carbondioxide injected improves the volumetric sweep efllcicncy as well as animproved displacement elilciency. This water-driven carbon dioxidemethod uses the injection of an optimum volume of carbon dioxidefollowed by water injection. As the carbon dioxide contacts the originaloil in place. it dissolves in the oil. increases the volume of thelatter. and reduces the viscosity of the oil. Thus, as the water bankadvances. it displaces the carbon dioxideenriched oil bank (originallysaturated with carbon dioxide). it also displaces the original oil inplace. A number of variations of this principle have been described andatcnted. However. they all suffer from the fact that they still leavevery substantial amounts of oil in the reservoir in either the ordinarywater-flooding or the water-driven carbon dioxide processes. After waterbreakthrough. the water-oil ratio increases until the economic limit isreached. in viscous oil reservoirs in which the water-driven carbondioxide process has been cmploycd, the area swept at water-breakthroughand at floodout is only a small portion of the reservoir. in such casesa large amount of oil is left unrecovered.

Accordingly. it is an object of my invention to provide a methodinvolving the use of an oil-soluble gas or mixturcs of such gaseswhereby increased quantities of oil can be recovered from a reservoirthrough the introduction of such gas or gases into said reservoir viaboth an injection and a production well or wells. it is another objectto provide an improved method of recovering oil from an undergroundreservoir by first injecting a suitable (iii 3,120,262 Patented Feb. 4,1964 ice oil-soluble gas into the ultimate producing well and thereafterintroducing said gas into said reservoir via an injection well until thegas-enriched oil thus formed constitutes at least a major portion of therecoverable oil. after which the system is subjected to a water-floodingoperation. it is still another object of my invention to provide asecondary recovery process that can be applied clilcicntly to viscousoil reservoirs of the type that heretofore have not been consideredsuitable for conventional watcrflooding operations.

FIGURE 1 is a diagrammatic representation of the mechanism involved inrecovering oil from a reservoir by means of the water-driven carbondioxide method.

FIGURE 2 shows the condition of the reservoir at the time ofwater-breakthrough at the producing well, (shown as well B). using thewater-driven carbon dioxide method.

FlGURE 3 illustrates the state of a hydrocarbon reservoir aftersomething less than percent of the hydrocarbon pore volume between wellsA and B has been filled with carbon dioxide.

FIGURE 4 is a representation of the reservoir after suliicicnt carbondioxide has been introduced via an injection well. (in this case wellA). to form a carbon dioxidc'curiched bank of oil extending from theproducing well to the injection well.

FIGURE 5 shows the swept area after the system illustrated in FIGURE 4has been waterfloodcd.

in a preferred embodiment of my invention. referring to the diagram ofFIGURE 3. carbon dioxide is injected into well I) (the ultimateproducing well), discontinuing injection at some point prior tobreakthrough into well A. The quantity of injected carbon dioxiderequired for this step corresponds to an amount not exceeding 50 percentof the hydrocarbon pore volume. When the volume of carbon dioxideexceeds about 50 percent of the reservoir hydrocarbon pore volume.breakthrough of carbon dioxide into well A. (the injection well).occurs. Accordingly. while the ultimate object of my invention can besecured with such quantities of carbon dioxide being injected via theproducing well, volumes of carbon dioxide incxccss of 50 percent in thisinitial injection step of my process are not only unnecessary. but arewasted and hence are uncconomical. During injection of carbon dioxideinto well ll, produccd oil is withdrawn from well A. When the desiredamount of gas has been introduced via well it. injection thereof ishalted and introduction of gas is begun in well A. if approximately 50percent of the hydrocarbon pore volume is introduced into each of wellsA and B, a pattern of gassaturated oil is formed similar in shape to theshaded area C, shown in FlGURE 4. While injection of carbon dioxide istaking place in well A. the fluid produced from well I) is carbondioxide-enriched oil. Carbon dioxide or other equivalent gas can easilybe separated from such oil and re-injcctcd into well A or into anotherwell for the purpose of producing additional gas-saturated oil.

it should also be pointed out that at the stage of operations wherecarbon dioxide or equivalent gas is being injected into the reservoirvia well A. the quantity of gas added at this time need not be enough toform a pattern of the type shown in FIGURE 4. it will be apparent fromthe foregoing description, and that which follows, that an improvedresult can be obtained even if the amount of gas added through well A isnot enough to touch the bank of carbon dioxide-enriched oil alreadyformed in the vicinity of well 8. The amount of gas first injected, i.e.well B. may vary from about 5 percent to about 50 percent of thehydrocarbon pore volume while the volume of gas injected during thesecond step. i.e., through well A, may range from a minimum of about 20percent to a maximum of about 50 percent of the hydrocarbon pore volume.In cases where a minimum gas volume of 20 percent is injected during thesecond step, larger amounts, i.e., 40 to 50 percent, of gas should beemployed for the initial injection step. Stated otherwise, the totalvolume of gas supplied during both injection steps prior towaterflooding should correspond to from about 60 to about 100 percent ofthe hydrocarbon pore volume in that portion of the reservoir lyingbetween the producing and injection wells, with the volume of gasinjected into any one well amounting to not more than 50 percent of saidhydrocarbon porevolume.

After the desired amount of gas has been llljCOlCd into the reservoirvia wells A and B, water is then introduced into the formation viaeither of wells A or B and gasenriehed oil recovered from the other ofthese wells until the economic limit is reached. The gain in oilrecovery obtained by this process is represented by the shaded area D ofFIGURE 5. By a comparison of the diagrams shown in FIGURES 2 and 5 it isapparent that the quantity of oil recovered by my process (FIGURE 5) isfrom A to A greater than that obtainable by use of the waterdrivcncarbon dioxide method (FIGURE 2).

it should be pointed out that the manner in which the process of myinvention is applied to a given reservoir may vary widely. For example,if desired. a conventional S-spot pattern can be used, with the ultimatewater injection well being in the center-or an inverted S-spotarrangement can be used, i.c., having the producing well in the center.

To enhance the solubility of the gas employed in the petroleum to berecovered, a mixture of said gas and one or more of the lower molecularweight hydrocarbons may be used. Natural gas containing typically 85 to90 percent methane snd from to percent C, to C and higher hydrocarbonsconstitutes a good medium with which to mix the drive gas of myinvention. in fact, a natural gas of the above indicated composition issuitable for use by itself as the gaseous component in my process. Also,the drive gas may be dissolved in a normally liquid low molecular weighthydrocarbon, such as-for example-naphthenes, aromatics, or paratiins,boiling below about 350' F. For best results, it is generallyrecommended that such solutions be saturated with respect to the drivegas. However, beneficial effects can be secured with the use ofsolutions containing lesser amounts of gas than the saturationconcentration. These solutions, of course, are to be used underconditions such that they remain predominantly in the liquid phase whilein the reservoir. Prevailing reservoir conditions determine the initialgas saturation that will be in accord with this requirement. Whilecarbon dioxide is an example of a suitable gas that can be dissolved inhydrocarbons for use in the manner indicated above, other oil-solublegases, either by themselves or in mixtures, can be dissolved into suchhydrocarbon and employed in accordance with the process of my invention.

Tire pressure employed in conducting the process described and claimedherein preferably should be the highest the particular reservoirinvolved can withstand without causing uncontrolled bypassing and/orfracturing of the formation. in the majority of instances pressures offrom about 1000 to about l0,000 p.s.i. are satisfactory. Higherpressures, of course, render the gas more dense, which in turn meansthat more gas is present to dissolve in a given volume of oil. 7

My invention is adapted particularly to heavy oil reservolrs having aviscosity of at least about 100 ccntipoises. it will be recognized thatby my invention it has now been made possible to water-flood reservoirswhich have been hitherto incapable of responding satisfacorily toconventional waterflooding or water-driven carbon dioxide processessince the latter procedures generally are applicable only to reservoirsin which the oil has a lower viscosity. Thus, I found that with crudeoils having a viscosity, for example of about 400 centipoises, whensaturated with carbon dioxide at room temperature, the viscositydecreases to about 22 ccntipoises. Oils having an initial viscosity of100 cenlipoises, when saturated with carbon dioxide under theseconditions, have a viscosity of 8 to 10 ccntipoises. it is apparent thatby such drastic reductio njn viscosity this change will be accompaniedby a very substantial increase in the reservoir pore vol ume filled withhydrocarbons. in essence this amounts to an appreciable increase in boththe horizontal sweep eiiiciency and the displacement ciliciencyexperienced in the subsequent ilooding step.

i claim:

l. in a process for recovering normally liquid hydrocarbons from anunderground reservoir thereof, having an injection well and a producingwell spaced apart from one another and extending into said reservoir,

the improvement which comprises injecting into said reservoir via one ofsaid wells an oil-soluble gas under pressure in an amount correspondingto not more than about percent of the hydrocarbon pore volume of thatportion of the reservoir between said wells,

discontinuing injection of said gas into said one of said wells andintroducing said gas into said reservoir via the other of said wellsuntil the quantity of said gas so introduced amounts to at least about20 percent but not more than about 50 percent of said hydrocarbon porevolume,

next injecting water into one of said wells, and

recovering gas-enriched hydrocarbons from the other of said wells.

2. The process of claim 1 in which the water injecting step is continueduntil the water-liquid hydrocarbon ratio of the produced fluids becomesuneconomical.

3. The process of claim 1 in which the oil-soluble gas employed iscarbon dioxide.

4. The process of claim 1 in which the oil-soluble gas is dissolved in ahydrocarbon boiling below about 350' F.

5. The process of claim 4 in which the oil-soluble gas is carbondioxide.

6. The process of claim 1 in which the water-injection step is carriedout in the injection well.

7. The process of claim l in which the initial gas injection step iscarried out in said producing well.

8. The process of claim l in which the waterflooding step is carried outby injection of water into said reservoir via the well into which saidgas was first introduced.

9. The process of claim 1 wherein the total quantity of said gasinjected into said reservoir amounts to from about percent to about l00percent of said hydrocarbon pore volume.

10. The process of claim I in which a mixture of oilsoluble gases isemployed.

soluble gases is employed.

12. The process of claim 1 in which the gas present in said recoveredgas-enriched hydrocarbons is separated therefrom and injected again intosaid portion of said reservoir.

13. The process of claim 1 wherein the viscosity of said liquidhydrocarbon is at least about ccntipoises.

ii. The process of claim 4 in which a mixture of oil-- References Citedin the tile of this patent UNITED STATES PATENTS

1. IN A PROCESS FOR RECOVERIN GNORMALLY LIQUID HYDROCARBONS FROM ANUNDERGROUND RESERVOIR THEREOF, HAVING AN INJECTION WELL AND A PRODUCINGWELL SPACED APART FROM ONE ANOTHER AND EXTENDING INTO SAID INTEROIR. THEIMPROVEMENT WHICH COMPRISES INJECTING INTO SAID RESERVOIR VIA ONE OFSAID WELLS AN OIL-SOLUBLE GAS UNDER PRESSURE IN AN AMOUNT CORRESPONDINGTO NOT MORE THAN ABOUT 50 PERCENT OF THE HYDROCARBON PORE VOLUME OF THATPORTION OF THE RESERVOIR BETWEEN SAID WELLS,